Transitions, rotational isomeric states

Cotton-Mouton effect), NMR chemical shift and coupling constants, the optical rotation of polarized light and correlation coefficients between different properties. Extensions to incorporate long-range interactions have also been elaborated11 and it has even been possible to adapt RIS theory for the description of the dynamics of transitions between rotational isomeric states.12,13... 

We have studied the thermochromism of fluorescence and show this behavior to be consistent with the rotational isomeric state model previously proposed to explain solution thermochromism in absorption (9,10). Weak, structured phosphorescence is observed from all polymers studied. The contrast between the structured phosphorescence and the narrow fluorescence is interpreted as evidence that the triplet state is the immediate precursor to photochemistry. Finally, the change in the fluorescence character in the aryl series on going from phenyl substitution to naphthyl substitution suggests a change in the nature of the transition from one involving mixed side chain-backbone states in the phenyl case to one which is primarily side chain-like for naphthyl-substituted polysilylenes. 

We have examined the emission spectra of a variety of polysilylenes as thin films and solutions. The solution fluorescence ther-mochromism provides evidence to support the rotational isomeric state model used to interpret the absorption spectrum. The structured character and low yield of phosphorescence in the alkyl polysilylenes suggest that the triplet is the immediate precursor to photochemical scission. The change in character of both fluorescence and phosphorescence on progressing from phenyl to naphthyl in the aryl series indicates that the transitions in the naphthyl polymers are principally ir—it. ... 

Table 1 Thermal pressure coefficients y, transition volumes A 7, and volume-dependent transition entropies ASy of n-alkanes and polyoxyethylene (POE) for the crystal-isotropic (Cl) phase transitions conformational entropy changes estimated by the rotational isomeric state approximation are included for comparison...

The above studies indicate that many conformational transitions occur as isolated transitions. For these motions, schematic representations like those shown in Fig. 7 are completely inadequate. Such representations assume that the rotational isomeric states (RIS) provide a reasonable basis set for understanding conformational dynamics. The observation that many transitions occur as isolated transitions cannot be explained within the RIS framework. In other words, many conformational transitions cannot be explained in terms of "cartoon pictures like Fig. 7. More general methods for discussing cooperativ-ity are examined in the next section. 

Stochastic TiKory of Conformational Transitions and Dynamk Rotational Isomeric State (DRIS) Ap oach... 

Relaxation 67,70,96,99,111,155 Reptation model 1,24,42 Resolution 14 Resonance NSE 20 Rheology 35,55 Rotational isomeric state 118 Rotational transitions 117 Rouse diffusion coefficient 28,42, 175 Rouse model 24-26,30-35,38, 117, 119, 142, 193,200 —, generalized 47 Rouse time 27 RPA 162, 163, 199... 

The chain molecule solution will be characterized by a set of rotational isomeric states. After the initial thermalization, this set will still be in a nonequilibrium state due to the overall shear deformation. For the internal torsional angle distribution to relax to equilibrium, it is necessary for the solution liquid structure to relax and the solvent molecule distribution to reach equilibrium. In a concentrated polymer solution, these processes are highly coupled. Rotational isomeric state changes depend on both internal potentials and the local viscosity and are often in the nanosecond range, well above the glass transition for the solution. Total stress relaxation cannot occur any faster than the chains can change their local rotational isomeric states. 

This overview of the range of time scales that are important for the description of a concentrated polymer solution emphasizes the complexity of these systems. More than 15 decades of time are often involved in the dynamics of concentrated polymer solutions. The dynamics can be organized into groups of relaxation times associated with specific types of relaxations. The local fluid structure and local rotational isomeric state dynamics are most closely associated with the phenomenon known as the glass transition and will be considered in more detail in CTiapter 8. 

In the following an outHne of the model and some of the assumptions used in the analysis are described [129]. The schematic sketch of the model is given in Fig. 25. In the model, conformational transitions are represented by a jump motion from a conformation (a rotational isomeric state) to another. In each rota-... 

The observed upfield shifts indicate changes in the molecular packing and the bond conformation as the sample is converted from a highly ordered crystal to the isotropic melt. In the melt, the Si resonance gives the fast exchange-averaged chemical shift for a dynamic equilibrium between different rotational isomeric states of the Si—O and Si—C bonds. The fact that the Si chemical shift is identical for the melt and the /x-phase demonstrates a dynamically disordered conformational state also below the isotropic transition. This is confirmed by Si spin-lattice relaxation experiments. The T1 time was 23 s for the melt at 330 K and 25 s for the /x-phase at 300 K. Thus, the motional state and the conformational equilibrium of the molecular segments remain very much the... 

The measurement of the dipole moments of copolymers and its analysis in terms of both sequence distribution and local chain configurations has received attention Modern computer aided analytical procedures provide in ght into the dependence of mean square dipole moment per residue on reactivity ratios, polymer composition and rotamer probabilities. One such calculation for atactic cc ly-(p-chlorostyrene-p-methylstyrene) has shovm that at constant composition, the dipole moment is quite sensitive to the sequence distribution and thus to the reactivity ratios. This dependence would be even more marked for syndiotactic chains. For cop61y(propylene-vinyl chloride) and copoly(ethylene-vinyl chloride) d le moments are again very sequence dependent, much more so than the diaracteristk ratio. It would appear that in copolymer systems dielectric measurements can be a powerful method of investigating sequence distributions. Two copolymers, p-dilcxo-styrene with styrene and with p-methylstyrene have been examined experimentally The meamrements were made on solid amorphous samples above the ass-rubber transition temperature (Tg) and they are consistent with the predictions of the rotational isomeric state model udi known reactivity ratios and rea nable replication probabilities . However, it is the view of this author that deduc-... 

Under these conditions, internal dynamics of the butane molecule may be envisaged as follows. Most of the time, the molecule is in either of the three conformational states and just vibrates about the respective energy minimum. From time to time, the molecule collects sufficient thermal energy so that the barrier can be passed over and the conformation changes. As the transitions take place rapidly compared to the times of stay near to a minimum, a sample of butane resembles a mixture of different rotational isomers . For each molecule there exist three rotational isomeric states . They are all accessible and populated according to the available thermal energy. 

If a polymer molecule is stretched out by applying forces to the end groups and then the forces are removed it returns to the initial coiled conformation. The reason for this behavior has already been mentioned The transition back to an isotropic coil increases the number of available rotational isomeric states and thus the entropy. The recoiling effect can also be expressed in mechanistic terms, by stating that, if the two endgroups of a polymer chain are held fixed at a certain distance, a tensile force arises due to the net moment transfer onto the ends. If, rather than keeping hold of the endgroups, two arbitrary points within a polymer molecule are kept at constant positions, a tensile force arises as well. 

---